isisd: fix isis_circuit_create()

isisd: fix isis_circuit_create()

Between the awkwardly managed CSM and the tacked-on IPv6 support, the
simplified logic to setup a circuit wasn't quite right.

Note that the API essentially allows creating a circuit without enabling
either IPv4 or IPv6.  This wasn't possible before and probably breaks
isisd in 'interesting' ways.  The CLI won't do this, so it's only an
issue when adding on other configuration mechanisms.

Reported-by: Martin Winter <mwinter@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>